The present invention relates to active matrix display devices, and more particularly to an active matrix liquid crystal display device (AMLCD) which is operable in a partial display mode and a method of driving the device.
AMLCDs utilising thin film transistors as switching devices for the picture elements are well known. An example is described in U.S. Pat. No. 5,130,829, the contents of which are hereby incorporated herein as reference material.
In many instances, it is desirable to minimise the power consumption of a display. This is particularly important in mobile devices such as mobile telephones or portable computers where reduced power usage extends the lifetime of the device""s battery. One way to operate a display in a low power mode is to only drive a xe2x80x9clivexe2x80x9d portion of the display area to show data, with the remainder of the display blank. An example of this approach is described in EP-A-0474231 wherein image data is compressed and displayed in a reduced display area using fewer of the row and column driver circuits of the display device.
It is an aim of the present invention to provide a method of addressing an active display in a partial display mode in an improved manner relative to known techniques, and a display device for implementing the method.
The present invention provides a method of driving an active matrix liquid crystal display in a partial display mode in which a live portion of the display is driven to display image data and a dormant portion is driven to display a substantially constant grey scale level output, the display comprising a set of row address conductors and a set of column address conductors, an array of picture elements each defined by a respective electrode connected to a respective address conductor of both sets and an opposing counter electrode, a column driver circuit for applying signals to the set of column address conductors, and a counter electrode driver circuit for applying a signal to the counter electrode which includes kickback correction corresponding to a predetermined grey scale level, wherein the method comprises applying a signal to each of the column address conductors associated with the dormant portion which comprises a combination of a signal substantially the same as the counter electrode signal and kickback correction, such that the resultant kickback correction applied to each of the picture elements associated with the dormant portion substantially corresponds to the grey scale level of the dormant portion.
This technique enables the or each dormant portion of the display to be driven to display a substantially constant output without generating a substantial DC across the picture elements of the dormant portion. Whilst a partial display mode can be achieved by turning off the drive to the column address conductors of the blank portion of the display which is not being used to display image content, this approach is problematic as the picture elements associated with those column address conductors will settle to a DC voltage. This DC across the picture elements may reduce the lifetime of the display and/or may result in image artefacts when a full display mode is restored.
According to a preferred embodiment of the method, the signal applied to each column address conductor associated with the dormant portion comprises a combination of the counter electrode signal and kickback correction substantially corresponding to the grey scale level of the dormant portion. The power consumption of the dormant portion can thereby be reduced. The column address conductors may be driven efficiently by using the same signal as is applied to the counter electrode.
To reduce the voltage swing applied to the column address conductors, and therefore the voltages which the column driver circuit needs to be able to handle, a counter electrode modulation drive scheme may be employed.
The invention further provides an active matrix liquid crystal display device operable in a partial display mode in which a live portion of the display is driven to display image data and a dormant portion is driven to display a substantially constant grey scale level output, the device comprising a set of row address conductors and a set of column address conductors, an array of picture elements each defined by a respective electrode connected to a respective address conductor of both sets and an opposing counter electrode, a counter electrode driver circuit for applying a signal to the counter electrode which includes kickback correction corresponding to a predetermined grey scale level, a column driver circuit for applying signals to the set of column address conductors, the signal applied to column address conductors associated with the dormant portion comprising a signal substantially the same as the counter electrode signal, and means for adding kickback correction to the signal applied to the column address conductors associated with the dormant portion, such that the resultant kickback correction applied to each of the picture elements associated with the dormant portion substantially corresponds to the grey scale level of the dormant portion.
Preferably, the adding means is operable to combine the counter electrode signal with a kickback correction signal substantially corresponding to the grey scale level of the dormant portion. The display device may include switching means for connecting column address conductors associated with the dormant portion to the output of the adding means.